Misfire on #1, P0300 after messing with motor mounts


Well-Known Member
Last week was very definitely TB 1, Chris 0....

I decided to finally tackle the motor mounts, and in the process, created a misfire on cylinder 1 that comes up as a P0300. I watched the live data and all the misfires appear on #1.

I loosened the motor mounts on both sides, and got the engine raised about an inch before I realized that my 21mm wrench for the frame mount bolts was missing (and probably up in NY with my father). Since loosening the frame mounts was how I was going to tackle this instead of taking the struts down, I figured that I'd just put things back together, run out and get another one.

The only electrical connectors I had mess with were to the fan housing and the air box. I never got far enough along to pull any more. When I jacked up the motor, none of the wiring harnesses was pulled or pinched (I was going *very* slowly).

When I put the engine back down, I made sure the mounts were still in the same position they had been in initially. Both were torqued to spec, as was the transmission mount. Started the truck up, and it was vibrating badly enough to stall. I hooked up the scan tool and watched it generate a P0300, but *all* the misfires are on #1. Just for the sake of argument I swapped the plug and coil (one at a time) from #1 to #2. The misfires stayed on #1. I followed the coil wiring back to see if I might have pinched something, and can't find any breaks.

Is it possible that breaking the old motor mounts loose disturbed them enough to cause the vibration? I know it's possible for a lot of vibration to be interpreted as a misfire, but I'm also getting a fuel odor that seems to also be from a misfiring cylinder.

I suppose the next thing I'll do is completely disconnect the #1 coil and see what effect it has, then look at the injector wiring, but thought I'd pop in here and see if someone had some other ideas as to what I'm missing while I'm taking a heat break.

Thanks for your time and input!



Maybe check the intake manifold bolts. Might have knocked it loose. Check the connectors for the injectors and coils.

And be sure you're dealing with the #1 cylinder when working on it. #1 is at FRONT (fan) end of engine. Ask me how I know...


Well-Known Member
Thanks, Mooseman!

Heh...I read your adventures while researching this problem. I labeled coils, connectors and highlighted the #1. I don't function well in the heat anymore, so I try to spell things out as much as possible for myself. I'll do things like go to swap ratchet extensions, have all of them in the same hand, and swap back in the one I was just meaning to take out.

Just finished going over the intake bolts, and everything was still torqued to spec. I'm going to go *back* over connectors, since it's so easy to miss things. In the process of lifting the engine I was really cautious about anything hitting, but it's possible I could have hit a connector while unbolting/reattaching.

Thanks again for the input!



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